U.S. Pat. No. 4,895,625 (Thoma et al.) discloses a method for galvanically or electrolytically depositing a protective coating on a structural component, for example, gas turbine blades that must be protected against hot gas corrosion. The protection layer is produced by suspending in an electrolytic solution a metal alloy powder of which the particles have a spherical configuration and a passivated surface. The concentration of the particles in the electrolyte is preferably smaller than 100 g/l, whereby a high insertion rate of up to 45% by volume can be achieved at relatively low costs and small technical efforts. The electrolyte forming the bath includes a matrix material of cobalt and/or nickel in which the above mentioned chromium and/or aluminum spherical particles are suspended for deposition on the component with the matrix in the galvanic process. After a coating of sufficient thickness has been galvanically deposited a heat treatment is performed for alloying the metals to form the protective coating.
It is the main purpose in the earlier disclosure to achieve a uniform high quality protective coating at small effort and expense. Such a coating can be achieved when the insertion rate exceeds 40% by volume of the alloying powder suspended in the metal matrix. However, even after the galvanically deposited layer on the structural component has been properly subjected to the heat treatment to form the alloy in the coating, there remain quality differences in different surface areas.
Experience has now shown that unexpected, localized quality changes can take place in the coating, especially with regard to the uniformity of the coating thickness throughout the surface of the structural component, and also with regard to the insertion rate of the metal alloying powder in the galvanically deposited matrix material. For example, substantial insertion rate differences have been observed between the top surface and the bottom surface of the structural component. Similarly, differences in the insertion rate may occur between the top surface and the lateral surfaces of the structural component.
Comparing tests have shown that surprisingly, vertical surfaces of structural components inserted into an electrolytic bath have a small insertion rate below 10% by volume of the metal alloying powder. This phenomenon has been observed even if the electrolytic bath itself is being rotated and even if gas bubbles are caused to flow through a stationary electrolytic bath.
Tests with structural components arranged predominantly horizontally in the electrolytic bath have shown that the downwardly facing surfaces of the components also had an insertion rate of the alloying powder smaller than 10% by volume of the metal alloying powder.